An optical fiber preform, which is used as the material of an optical fiber, consists of a core preform constituting the core of an optical fiber and a cladding layer provided in the outer periphery of the core preform and constituting the cladding of the optical fiber. By drawing this optical fiber preform, it is possible to produce an optical fiber.
According to one aspect of the present invention, there is provided an optical fiber preform comprising:
a core preform constituting a core of an optical fiber; and
a cladding layer formed in the outer periphery of the core preform and constituting a cladding of the optical fiber,
wherein the optical fiber preform is comprised of a large-sized optical fiber preform having an outer diameter of 100 mm or more, and
wherein the optical fiber preform has in a base end region, which is on the upper side when it is suspended for a drawing process to produce an optical fiber, a shoulder portion sticking out beyond a middle portion, and has, in a tip end portion on the lower side, a large diameter portion whose outer diameter is larger than that of the middle portion.
Further, according to another aspect of the present invention, there is provided a method for producing an optical fiber preform comprising:
a step of forming, in the outer periphery of a core preform that constitutes a core of an optical fiber, a cladding layer constituting a cladding layer of the optical fiber to thereby prepare a soot body; and
a sintering step to heat the soot body for vitrification,
wherein, in the sintering step, the soot body is suspended and heated sequentially from the lower end toward the upper end thereof, the weight of the lower end portion being utilized when a middle portion of the soot body is being heated to draw the middle portion so that its outer diameter may become smaller than that of the other portions, and
wherein, when heating the upper portion of the soot body, the heating temperature is lowered to thereby leave a non-sintered portion in the upper portion of the soot body.